In an optical information recording medium, a compact disc (CD), a laser disc (LD), or the like has been widespread. In recent years, a DVD having a recording density that is seven or more times as large as that of the CD has been put into practical use. As for the DVD, development is being made as a rewritable recording/reproducing medium besides a read only ROM (DVD-ROM) in which information has directly been written on a board. The realization of the practical use of the DVD is being examined also as an RAM for a computer (DVD-RAM).
In a DVD, a high density recording has been accomplished by using a laser beam having a shorter wavelength of about 650 nm than that of the laser (780 nm) used in a CD or the like. In order to handle information of a large capacity such as computer graphics or the like, however, it is necessary to accomplish a further high recording density that is 1.5 to 2 times as large as the above density. To accomplish it, development of semiconductor lasers of green to blue of further short wavelengths (wavelengths: 520 to 410 nm) is being made.
A super resolution film can be mentioned as another high recording density technique. The super resolution film is a film which is formed on a lower surface of a recording medium and a high recording density can be accomplished by reducing a beam spot of incident light transmitted through the film.
One of mechanisms of a super resolution effect is a satural absorption phenomenon which is a phenomenon realized by using such nonlinear optical characteristics that the super resolution film transmits light having an intensity that is equal to or larger than its satural absorption amount and absorbs light having an intensity below the satural absorption amount. Since a spatial intensity of the laser beam which is used for reading or writing has a Gaussian distribution, when the beam passes through the super resolution film, the light at a bottom portion having a low intensity is absorbed by the super resolution film and the light at a center portion having a high intensity is transmitted. Therefore, a beam diameter after the transmission can be reduced.
At present, as such a super resolution film, an organic film of the phthalocyanine system, materials (compounds) of the chalcogenide system, or the like as shown in JP-A-8-96412 or the like can be mentioned. Besides them, such a trial that, as the same organic material, a thermochromic material disclosed in JP-A-6-162564 or a photochromic material disclosed in JP-A-6-267078 is used as a super resolution film is also known.
However, each of the materials as mentioned above has problems in terms of the reliability, productivity, and the like. In the organic film, since an energy density of the beam is locally very high upon recording or reading, if the recording or reproducing operation is repetitively performed, there is a fear that the film deteriorates gradually. Therefore, it is difficult to guarantee the sufficient number of times of the recording or reproducing operation under a severe use environment as in case of an RAM for a computer or the like. Since chalcogenide is chemically unstable, it is difficult to obtain a long guaranteeing period.